As described in cited Patent Documents 1 and 2, the basic structure of a flow path switchover device has a tube assembly including a main tube member and a branch tube member. A changeover damper is provided in the tube assembly to swing about a rotating shaft as the supporting point. The outer circumferential edge of the changeover damper has a packing.
The rotating shaft is rotated by adequate means to swing the changeover damper. The packing of the swinging changeover damper is in pressure contact with an inner wall face of the tube assembly to block the joint of the main tube member and the branch tube member and allow connection between an inlet and an outlet of the main tube member or to block the middle of the main tube member and allow connection between the inlet or the outlet of the main tube member and the branch tube member. The flow path is changed over in this manner. The packing controls the amount of air leakage to a relatively low level even in the high blower pressure condition, compared with a conventional flow path switchover device, for example, a cast ball valve.
The respective tube members are produced as pressed sheet metal parts, which are mutually connected by welding. This structure is lighter in weight than the conventional flow path switchover device, for example, the cast ball valve, and attains the easy processing and good cost performance.    Patent Document 1: Japanese Utility Model Publication Gazette No. S-63-33858    Patent Document 2: Japanese Utility Model Publication Gazette No. H02-19673